Enable SystemBackupAgent to integrate with metrics.

Pass a logger instance to BackupHelpers in SystemBackupAgent.

Bug: 296844513
Test: atest -v SystemBackupAgentTest; manual: override flag value, add log events to some helpers in the SystemBackupAgent, check that the correct agent logs are printed in the backup dumpsys.

Change-Id: I2d14d49516849f4c992e8180baf4c3d431326a26
diff --git a/services/backup/flags.aconfig b/services/backup/flags.aconfig
index 549fa36..4022e33 100644
--- a/services/backup/flags.aconfig
+++ b/services/backup/flags.aconfig
@@ -10,6 +10,15 @@
 }
 
 flag {
+    name: "enable_metrics_system_backup_agents"
+    namespace: "backup"
+    description: "Enable SystemBackupAgent to collect B&R agent metrics by passing an instance of "
+            "the logger to each BackupHelper."
+    bug: "296844513"
+    is_fixed_read_only: true
+}
+
+flag {
     name: "enable_max_size_writes_to_pipes"
     namespace: "onboarding"
     description: "Enables the write buffer to pipes to be of maximum size."